1. Field of the Invention
The present invention relates to a semiconductor device having a circuit that is formed of a thin film transistor (hereinafter referred to as a TFT), and a method of manufacturing thereof. Specifically, the present invention relates to an electronic appliance mounted with an electro-optic device or a light emitting display device having an organic light-emitting element typified by a liquid crystal display panel as a component.
It should be note that the term “a semiconductor device” indicates all kinds of devices that can behave by utilizing semiconductor characteristics, and includes all of an electro-optic device, a semiconductor circuit, and an electronic appliance.
Further, the present invention relates to a method of manufacturing a base material including a protective film, which is superior in barrier properties.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (a TFT) by using a semiconductor thin film (with a thickness of about from several nm to several hundreds nm) that is formed over a substrate having an insulated surface has been attracting attention. The thin film transistor has been widely applied in electronic devices such as an IC and an electro-optic device. In particular, development related to the thin film transistor as a switching element of an image display device has been hurried.
Various applications utilizing such a image display device have been expected, and particularly, application to a portable device has been attracting much attention. As a substrate for forming the TFTs, a glass substrate and a quartz substrate has been widely used now, however, these substrates have some drawbacks of being fragile and heavy. Further, these substrates are unsuitable for mass-production since the surface area thereof is difficulty enlarged. Therefore it has been attempted that an element including TFTs is formed over a substrate having flexibility as typified by a flexible plastic film.
In the case of using the flexible plastic substrate, however, the maximum temperature of the process should be lowered since the plastic film has low heat resistance. Therefore, it has been impossible to form a TFT having as good characteristics as that formed over a glass substrate. Thus, a high-performance liquid crystal display device or light emitting element using a plastic film has not been realized yet.
Further, the plastic film has a drawback of poor barrier properties. Even when a protective film having good barrier properties is provided in order to compensate the drawback, since the plastic film has low heat resistance, it is necessary to reduce the maximum temperature of a film formation process. Therefore, a favorable protective film cannot be obtained. Accordingly, a highly reliable electronic device using the plastic film has not been realized yet.
Particularly, since an EL display device (panel) using an EL element is sensitive to moisture, it is assumed that far superior gas barrier properties are required.
With respect to the EL display device (panel) using an EL element, moisture penetrating into the device causes a serious deterioration in the reliability of the EL display device, which further causes dark spots, shrinkage, and deterioration in luminance from the periphery of a light emitting display device. The dark spots are phenomena in which the luminance is partially decreased (including a phenomenon of non-light emission), and caused in the case where a hole is formed in an upper electrode. The shrinkage is a phenomenon in which the luminance is deteriorated from edge of pixels.
Consequently, development related to a display device having a structure for preventing the above-mentioned deteriorations in the EL element has been carried out. For example, there is a technique for preventing the deterioration of the EL element as follows (for example, see patent document 1): an EL element is encapsulated in an airtight container so as to protect the EL element from outside air, and a desiccant is provided apart from the EL element in the airtight space.
Further, there is another method for preventing the deterioration of the EL element as follows (for example, see patent document 2): a sealing material is formed on an insulator with an EL element formed thereon, an airtight space surrounded by a cover member and the sealing material is filled with a filler that is composed of resin and the like so that the EL element is shielded from the outside air.
Furthermore, the patent document 3 discloses a structure in which an electrode over a luminescent layer is coated with a water-shedding protective film, and a plate such as glass is attached firmly thereon.
Also, the patent document 4 discloses a structure in which a light curable resin is applied over an EL element and cured by being irradiated with light to seal a two-layered film, thereby protecting the EL element mechanically.    Patent document 1: Japanese Patent Application Laid-Open No. Hei 9-148066    Patent document 2: Japanese Patent Application Laid-Open No. Hei 13-203076    Patent document 3: Japanese Patent Application Laid-Open No. Hei 10-106746    Patent Document 4: Japanese Patent No. 2793048
When the film thickness of a passivation film made from an inorganic insulating film is increased in order to prevent intrusion of impurities into a semiconductor element, e.g., intrusion of moisture and the like into an EL element, stress thereof is also increased, and hence, cracking is easily caused.
In the case where a protective film is formed over an entire surface of a substrate with elements formed thereon by PCVD or application, an external extraction terminal portion is also covered with the protective film. Therefore, it has been necessary that the substrate be selectively etched to expose the external extraction terminal portion. Meanwhile, in the case of using the sputtering technique, although the protective film can be selectively formed with use of a metal mask, there is a risk of damaging the elements formed over the substrate due to the sputtering.
Moreover, when a glass substrate is used as a sealing substrate and adhered to a substrate with elements formed thereon to encapsulate the elements, there have been drawbacks of being easily broken and heavy.